Polyester has long been recognized as a desirable material for textile applications including garments, upholstery and numerous other uses. The processes for manufacture of polyester are relatively well known and straight forward to those knowledgeable in the art, and fibers made from polyester can be appropriately woven or knitted to form textile fabrics. Polyester fibers can be blended with other fibers such as wool or cotton to produce fabrics which have the enhanced strength, durability and memory aspects of polyester and retain many of the desirable qualities of the natural fiber with which the polyester is blended.
As with any fiber, the particular polyester fiber from which any given fabric is formed must have properties suitable for the end use of the fabric. In many applications such as sleepwear fabrics, draperies, and bedspreads, for example, it is desirable for the textile fabric to have the property of flame resistance. Flame resistant fabrics are defined as fabrics that will self extinguish when an ignition source is removed. Requirements are set forth in various tests including the NFPA 701-1977.
One technique for attaining flame resistance of fabrics of polyester fiber is to modify the polyester with carboxyphosphinic acids. Such modified polyester fibers and their use in fabrics are disclosed in U.S. Pat. Nos. 3,941,752; 4,033,936; and RE 30,783. In fact, these patents describe flame resistant linear polyesters which are modified polyesters consisting of dicarboxylic acid derived components, diol derived components and phosphorus containing chain members, the latter being derived from structural units of the formula ##STR1## which constitute about 1-20 mole percent of the acid component of the polyester. In this formula R is a saturated open-chained or cyclic alkylene, arylene, or aralkylene having from 1-15, preferably from 2-10 carbon atoms and R is an alkyl radical having up to 6 carbon atoms or an aryl radical or an aralkyl radical as described in U.S. Pat. No. 3,941,752. Such modified polyester fibers are currently available from Hoechst Celanese Corporation.
Although fabrics containing polyester fibers as described above provide flame resistance, such fibers have an undesirable propensity upon prolonged use to exhibit small, compact groupings of entangled fibers (i.e., fuzzballs) on the fabric surface. Such fiber groupings commonly are termed "pills" and tend to form and to tenaciously adhere to the surface of the fabric as the fabric encounters surface abrasion during normal use. The aesthetic appearance of fabric accordingly may be adversely influenced by these relatively small groupings of entangled fibers which are retained on the surface of the fabric.
Heretofore, it has been believed that the prevalence of such pills can be traced to the relatively high strength of the synthetic fibers present in the fabric. For instance, the pills may be more or less permanently attached to the fabric surface by one or more synthetic polymer fibers extending out of the fabric which will resist breakage as the surface abrasion continues. This theory of pill formation is supported by the significant lower level of the retention of undesired fuzzballs on the surface of fabrics consisting solely of cotton fibers following the same surface abrasion conditions. It is believed that the entangled cotton fibers which form at the surface of a fabric more readily break away since the cotton fibers are of an inherently lower strength.
This pilling problem may be observed in fabrics formed in whole or in part from polyethylene terephthalate (PET) fibers. Pills commonly are observed on fabrics formed from blends of cotton and PET staple fibers following use in service and during the cleaning process including laundering or dry cleaning. While pills may be observed on fabrics having a wide variety of constructions, they are more commonly observed on loosely constructed fabrics, particularly knitted fabrics.
One approach heretofore proposed to reduce the pilling of fabrics is to reduce the tenacity (or strength) of the PET fibers by using a low molecular weight (measured as intrinsic viscosity) PET polymer. Low intrinsic viscosity provides a general indication of reduced polymeric chain length and leads to fibers having a lesser strength. Accordingly, when such entangled fibers become free on the surface of the fabric following abrasion, the fibers tend to cleanly break away and not hold a fuzzball at the surface of the fabric.
It has been found, however, that when this approach is followed, it is more difficult to form the fibers because of the resultant low melt viscosity of the polymer, and subsequently to process the PET fibers into a fabric using standard processing conditions because of the reduced tenacity. Such reduced tenacity leads to premature fiber breakage and consequential processing problems. These processing problems can only be partially overcome by gentler textile processing conditions, which are costly. Further, it has also been known that for PET, an intrinsic viscosity reduction as low as is tolerable from the standpbints of fiber formation and processing does not resolve the problem of pilling sufficiently to satisfy the market.
It is also well known to make modified chain branched polyester polymers in order to reduce pilling in the fabric. Chain branching of polyester fibers can be accomplished by inclusion of chain branching agents such as tetrafunctional chain branching agents, in particular, pentaerythritol and tetraethyl orthosilicate.
U.S. Pat. No. 3,576,773 discloses low pilling PET fibers containing trifunctional or tetrafunctional branching agents. Pentaerythritol is listed as a branching agent and is shown used in Example 5. It is known that pentaerythritol is a "permanent" branching agent, maintaining the bonds throughout processing of the fiber and the fabric.
U.S. Pat. No. 3,335,211 discloses low pilling modified PET fibers made from polymers in the presence of a polycondensation catalyst of antimony or titanium by adding an oxysilicon compound prior to melt spinning such as tetraethyl orthosilicate.
Tetraethyl orthosilicate (TES) forms a non-permanent chain branching susceptible to hydrolysis, especially under acidic conditions. When the TES bonds are broken by hydrolysis, the melt viscosity lowers, making the polymer difficult or impossible to process. However, if care is taken to protect TES modified polymers from moisture, much of the chain branching provided by the TES remains when these polymers are remelted in the course of being formed into fibers. Thus, a high molecular weight (high intrinsic viscosity) is maintained throughout the fiber formation process. This provides a high melt viscosity which facilitates fiber formation, and the high fiber strength needed for efficient processing of the fibers into fabric. Subsequently, under the hot wet acidic conditions used in dyeing the fabric, the TES chain branching sites are cleaved by hydrolysis. The resultant reduced molecular weight (reduced intrinsic viscosity) of the polymer in the fibers of the fabric reduces fiber strength, which is believed to lead to the reduced pilling observed.
In attempts to make low pilling flame resistant fibers, PET polymers modified with both a carboxyphosphinic acid and TES have been made. Such polymers could be readily made by conventional techniques, and they exhibited the chain branching expected based upon their TES content. However, upon remelting to form them into fibers, all chain branching was immediately lost, with a commensurate reduction of molecular weight as measured by intrinsic viscosity. As would be expected with such a loss of molecular weight, forming fibers from these polymers was too difficult to be practiced.
There remains a need to develop a processable polyester fiber having flame resistance and low pilling properties while also maintaining the other properties desired in the resulting fabric, especially the aesthetic properties of the fabric resulting from the polyester fiber.